If it's an 85% chance of getting grounded then it's a 0.85 chance. On top of that, getting extra chores if 50% or 0.50 so multiply them together to get a probability of 0.425 or 42.5%.
It works because 7 x 9 x 10 is the same thing as 7 x 90, since 9 x 10 is just 90. So basically he is just writing more but the answer will still be the same.
